Hi Kate
Not having overlooked the fact you believe William's father to be George, Mike has found the marriage of another William John Bufton in March 1901, Portsmouth to Ellen Elizabeth Dimmer (there are others listed on the record page for this marriage but having checked the 1901 census I see the other two individuals were man and wife, so by process of elimination I have found William John Bufton did indeed marry Ellen Elizabeth Dimmer).
Mike also found the marriage of a George Bufton but this was back in 1859 (31st January) to Frances Johnson. As Mike says, these could have been William John's parents (your William's grandparents). This could be proved by checking earlier census images.
Kay
Manager